LED/phosphor-LED hybrid lighting systems

ABSTRACT

A hybrid lighting system for producing white light including at least one light emitting diode and phosphor-light emitting diode. The hybrid lighting system exhibits improved performance over conventional LED lighting systems that use LEDs or phosphor-LEDs to produce white light. In particular, the hybrid system of the invention permits different lighting system performance parameters to be addressed and optimized as deemed important, by varying the color and number of the LEDs and/or the phosphor of the phosphor LED.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/453,420, filed Dec. 2,1999 now U.S. Pat. No. 6,513,949.

FIELD OF THE INVENTION

This invention relates to light emitting diode (LED) lighting systemsfor producing white light, and in particular to hybrid LED lightingsystems for producing white light comprised of LEDs and phosphor-LEDs.The hybrid lighting system exhibits improved performance overconventional LED lighting systems that use LEDs or phosphor-LEDs toproduce white light.

BACKGROUND AND SUMMARY

Conventional LED lighting systems for producing white light typicallycomprise either LEDs or phosphor-LEDs. Lighting systems which use LEDsproduce white light by combining various combinations of red, green, andblue LEDs. Phosphor-LED based lighting systems produce white light byusing one or more various luminescent phosphor materials on top of ablue light LED to convert a portion of the emitted blue light into lightof a longer wavelength.

Lighting systems which use LEDs to produce white light are moreefficient at the package level than lighting systems which usephosphor-LEDs. However, high quality white light is more difficult toachieve in LED based lighting systems. This is because LEDs manufacturedto optimize total lighting system performance and production typicallymust be combined in an undesirably large integral number of LED chips toprovide the requisite quantities of red, green and blue light whenoperated at full rated power. Moreover, the LED chips must be fabricatedin different sizes to achieve the proper balance thus, increasing theproduction costs of the system. Since the green and blue LED chips aremanufactured in the same AlInGaN technology, there are fabrication andcost advantages to making these chips the same size, and reasonablylarge.

There are other limitations associated with LED based lighting systems.Existing green LEDs operating at the very desirable light spectralwavelength of about 550 nm are very inefficient. The highestluminous-efficacy green LED operates at a less desirable light spectralwavelength of about 530 nm. Further, currently available efficient LEDsmake good color rendering difficult to achieve. Good color rendering ispossible, but places constraints on specific choices of LEDs.

Additionally, mixing LEDs to produce white light entails material andparticularly efficiency costs. More specifically, many highly collimatedmixing schemes are binary in that they mix two LEDs at a time. LED basedlighting systems typically use three and four LEDs and thus, require twostages of mixing. Unfortunately, each stage of mixing has an efficiencycost which significantly lowers the performance of the system.

As alluded to earlier, it is easier to produce white light withphosphor-LED based lighting systems as compared with LED based lightingsystems because phosphor-LEDs do not require mixing and have lowermaterial costs (they are inherently mixed). However, they are lessefficient by a factor of about two at the package level than LED basedlighting systems because of quantum deficits and re-emissionefficiencies.

Accordingly, there is a need for a lighting system which combinescertain aspects of LED and phosphor-LED based lighting systems toachieve benefits beyond either system.

Accordingly, a lighting system for producing white light is disclosedthat includes at least one LED and a phosphor LED disposed adjacent tothe at least one LED.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature, and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection withaccompanying drawings wherein:

FIG. 1A is a sectional view of a typical LED used in the lighting systemof the invention;

FIG. 1B is a sectional view of a typical phosphor-LED used in thelighting system of the invention;

FIG. 2 is a schematic diagram depicting a first embodiment of thelighting system of the invention;

FIG. 3 is a schematic diagram depicting a second embodiment of thelighting system of the invention

FIG. 4 is a schematic diagram depicting a third embodiment of thelighting system of the invention;

FIG. 5 is a schematic diagram depicting a fourth embodiment of thelighting system of the invention;

FIG. 6 is a schematic diagram depicting a fifth embodiment of thelighting system of the invention; and

FIG. 7 is a schematic diagram depicting a sixth embodiment of thelighting system of the invention.

It should be understood that these drawings are for purposes ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

The hybrid lighting system of the invention generally comprisesselectively combining one or more certain LEDs with a phosphor-LEDconsisting of a blue LED and at least one phosphor which emits atcertain light spectral wavelength (color), to produce white light.

FIG. 1A schematically depicts a typical LED 10 used in the invention.The LED 10 is conventionally constructed using standard AlInGaN orAlInGaP processing and comprises an LED chip II mounted in a reflectivemetal dish or reflector 12 filled with a transparent epoxy 13.

FIG. 1B schematically depicts a typical phosphor-LED 14 used in theinvention. This LED 14 is substantially identical in construction to theLED of FIG. 1A, except that the epoxy 18 filling the reflector 16contains grains 19 of one or more types of luminescent phosphormaterials mixed homogeneously therein. The phosphor grains 19 convert aportion of the light emitted by the LED chip 15 to light of a differentspectral wavelength.

A primary advantage of the hybrid system of the invention is that itpermits different lighting system performance parameters to be addressedand optimized as deemed important by varying the color and number of theLEDs and/or the phosphor of the phosphor-LED. In particular, the systemof the present invention can address and achieve higher availablelumens-per-chip averages via balancing red and green and blue lightcomponents; smaller integral numbers of LEDs at balance; improved colorrendering; and more efficient mixing via the phosphor-LED. This in turnadvantageously permits the manufacture of various types of productswhich are optimized for various applications using LED chipsmanufactured with standard AlInGaN and AlInGaP processing.

Each of the following embodiments illustrate how one or more of theabove performance advantages can be achieved using the hybrid system ofthe invention. FIG. 2 schematically depicts a first embodiment of thehybrid lighting system of the invention denoted by numeral 20. Thesystem 20 comprises a red LED 22 that emits light having a spectralwavelength of about 610 nm and a phosphor-LED 24 consisting of a blueLED that emits light having a spectral wavelength of between about 450nm and 470 nm, and a phosphor material that converts a portion(typically about 50%) of the blue light to green light having a spectralwavelength of about 550 nm. This embodiment of the inventive systemprovides reasonable power balance and achieves an overall systemefficiency comparable with or exceeding conventional LED based lightingsystems because it eliminates the need for a less efficient green LED(typically emits light having a spectral wavelength of about 530 nm) asemployed in conventional LED based lighting systems (the phosphor LEDcould be fundamentally more efficient than the green LED chip because ithas better access to the 550 nm range and thus, has a lumen-per-wattadvantage). Moreover, the two LEDs 22, 24 of this embodiment of thesystem 20 are easier and more efficient to mix, and require less complexLED drive electronics than conventional lighting systems which typicallyemploy three or four LEDs. More specifically, there are many mixingschemes for mixing LEDs. In virtually all of these schemes, less mixingis desirable. For example, in binary mixing schemes, only one mixingstage is required to mix the two LEDs 22, 24 of the inventive system 20compared with two mixing stages required for conventional LED basedlighting systems. Hence, approximately half the mixing losses ofconventional LED based lighting systems are incurred with thisembodiment of the system. Therefore, significantly higher overall systemefficiency can be realized in this embodiment of the system whencompared with conventional LED based lighting systems. Color renderingis adequate owing to the 550 nm light provided by the phosphor-LED 24although the color point can only be adjusted along a line.Consequently, control over color temperature is only possible at thedesign point. However, such an embodiment of the invention is especiallyuseful for producing a fixed white light in lower cost lighting systemapplications where less expensive LED drive electronics are desirableand truly versatile control over color temperature is not significantlyimportant.

FIG. 3 schematically depicts a second embodiment of the hybrid lightingsystem of the invention denoted by numeral 30. The system 30 comprises ared LED 32 (emits light having a spectral wavelength of about 610 nm), agreen LED 34 (emits light having a spectral wavelength of about 530 nm)and a phosphor-LED 36 consisting of a blue LED (emits between ˜450 nmand ˜470 nm) and a phosphor material that converts a portion (about 50%)of the emitted blue light to red light (about 610 nm). In comparison toconventional LED based systems which typically produce insufficientquantities of red light and excessive quantities of blue light, thephosphor LED 36 used in this. embodiment of the inventive systembalances the deficiency in red light output and attenuates the bluelight output thereby providing good color balance with only three LEDs.Color rendering and color-temperature control is comparable toconventional LED based lighting systems. In particular, the use of threeLEDs can provide maximum lumen content while permitting the colortemperature to be freely adjusted, since the red content of thephosphor-LED 36 is sufficiently small to allow a balance to be struck bymerely raising and lowering the brightness of the three LEDs viaadjustments in the drive currents.

FIG. 4 schematically depicts a third embodiment of the hybrid lightingsystem of the invention denoted by numeral 40. The system 40 comprisestwo red LEDs 42, 44 (emits at ˜610 nm), a green LED 46 (emits at ˜530nm), and a phosphor-LED 48 consisting of a blue LED (emits between ˜450nm and ˜470 nm) and a phosphor material that converts a portion of theblue light to green light having a spectral wavelength of about 550 nm.Since the phosphor has a power conversion efficiency of about 50%,sufficient quantities of blue and green light are generated by thephosphor-LED 48 thereby providing excellent balance and color renderingat the design point owing to the presence of the 550 nm light.Additionally, the 550 nm light produces much greater lumen content,thereby providing this embodiment of the system with a higher lumenoutput, even when the phosphor's conversion energy loss if factored in.

FIG. 5 schematically depicts a fourth embodiment of the hybrid lightingsystem of the invention denoted by numeral 50. The system 50 issubstantially identical to the system of the third embodiment in that itcomprises two red LEDs 52, 54, a green LED 56 and a phosphor-LED 58 thatemits blue light and green light. However, the two red LEDs 52, 54 emitlight at two slightly different spectral wavelengths (˜610 nm and ˜595nm). The red LED emitting at about 595 nm produces a red color that isorange-amberlike to provide further improvements in color rendering.

FIG. 6 schematically depicts a fifth embodiment of the hybrid lightingsystem of the invention denoted by numeral 60. The system 60 issubstantially identical to the system of the third embodiment in that itcomprises two red LEDs 62, 64, a green LED 66 and a phosphor-LED 68.However, the phosphor-LED 68 consists of a blue LED with a firstphosphor material that emits at a light spectral wavelength of about 550nm to convert a portion of the blue light to green light and a secondphosphor material that converts a portion of the remaining blue lightnot converted by the first phosphor to an amber or yellow-green light.This embodiment exhibits further improvements in color rendering and istherefore useful in applications requiring maximized color rendering.

FIG. 7 schematically depicts a sixth embodiment of the hybrid lightingsystem of the invention denoted by numeral 70. The system 70 combinesthe features of the fourth and fifth embodiments and thus, comprises afirst red LED 72 (emits at ˜610 nm), a second red LED 74 (emits at ˜595nm), a green LED (emits at ˜530 nm) 76, and a phosphor-LED 78. Thephosphor LED consists of a blue LED (emits between ˜450 nm and ˜470 nm)and a first phosphor material that emits at a light spectral wavelengthof about 550 nm to convert a portion of the blue light to green lightand a second phosphor material that converts a portion of the remainingblue light not converted by the first phosphor, to an amber oryellow-green light. This embodiment is also especially useful inapplications requiring optimized color rendering.

While the foregoing invention has been described with reference to theabove embodiments, various modifications and changes can be made withoutdeparting from the spirit of the invention. Accordingly, all suchmodifications and changes are considered to be within the scope of theappended claims.

What is claimed is:
 1. A lighting system for producing white light, thesystem comprising: at least one light emitting diode; and aphosphor-light emitting diode disposed adjacent to the at least onelight emitting diode.
 2. The lighting system according to claim 1,wherein the phosphor-light emitting diode emits at least two differentcolors of light.
 3. The lighting system according to claim 1, furthercomprising at least a second light emitting diode.
 4. The lightingsystem according to claim 3 wherein the phosphor-light emitting diodeemits at least two different colors of light, one of the at least twocolors being at least substantially identical to the color emitted by atleast one of the light emitting diodes.
 5. The lighting system accordingto claim 3, wherein the light emitted from the light emitting diodes areof the same general color but have different spectral wavelengths. 6.The lighting system according to claim 1, further comprising second andthird light emitting diodes.
 7. The lighting system according to claim6, wherein the light emitted from two of the three light emitting diodesare of the same general color but have different spectral wavelengths.8. The lighting system according to claim 7, wherein the phosphor-lightemitting diode emits at least three different colors of light.
 9. Thelighting system according to claim 6, wherein the phosphor-lightemitting diode emits at least three different colors of light.
 10. Thelighting system according to claim 6, wherein the phosphor-lightemitting diode emits at least two different colors of light.
 11. Thelighting system according to claim 1, wherein the at least one lightemitting diode emits red light.
 12. The lighting system according toclaim 11, wherein the phosphor-light emitting diode includes a bluelight emitting diode that emits blue light and at least one phosphorthat converts at least a portion of the blue light to one of a greenlight and a red light.
 13. The lighting system according to claim 12,further comprising at least a second light emitting diode which emitsgreen light.
 14. The lighting system according to claim 13, furthercomprising a third light emitting diode which emits red light.
 15. Thelighting system according to claim 14, wherein the spectral wavelengthof the red light emitted from the first light emitting diode isdifferent from the spectral wavelength of the red light emitted from thethird light emitting diode.
 16. The lighting system according to claim11, further comprising at least a second light emitting diode whichemits green light.